Answer: I think you're dealing with either mealy bugs or scale insects (it's difficult to know for sure without seeing the pests). Ants protect these insects because they exude a sweet, sticky substance called honeydew which the ants use as a food source. I suggest scraping a few of these pests off and taking them to your local nursery professional for identification. They can determine just what the pest is and suggest a control measure. The black splotches are probably sooty mold, the result of the sticky honeydew remaining on the leaves for extended periods of time. The sooty mold will wash off with water. Once you've eliminated the insect pests, the sooty mold will be eliminated, as well.
Best wishes with your grapefruit tree!
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